1. Field of the Invention
The present invention relates to resistive bodies fabricated on a substrate. In particular, the present invention is a slotted resistive body adapted for use on integrated circuits and which can be trimmed to a desired resistance value.
2. Description of the Prior Art
Many analog and other integrated circuits require precise matching of the resistance values of resistors fabricated thereon to achieve desired overall circuit precision. The "natural" level of matching for integrated circuit resistors (i.e., that achievable by controlling parameters of the manufacturing process) is approximately 0.1-0.3%. For some circuits such as high-precision analog-to-digital and digital-to-analog converters, this degree of precision is inadequate. To produce such high-precision devices, various forms of post-fabrication trimming have been devised to adjust the resistance value of one resistor of the matched pair. Known techniques include laser trimming or cutting, Zener-zapping, and metal-link cutting and blowing.
Other integrated circuits include individual resistors which must be trimmed to an absolute resistance value. In applications of these types, untrimmed accuracies on the order of only 15-20% are typical due to the wide manufacturing variations in sheet resistance of the integrated circuit.
Laser trimming involves the use of a laser beam to alter the shape of a resistor region and thereby bring its resistance to a desired value. At present, many kinds of trim geometries are used for this purpose. "Top Hat", "L-cut" and other trim patterns are common.
Serious problems arise from aging and annealing affects resulting from laser trimming techniques. The "partially zapped" material along the edge of the cut trim path often has different properties from undisturbed material, and its resistance ages (anneals) at a different rate than the body of the resistor. This can give rise to a situation where a resistor pair which was initially trimmed to a precise ratio exhibits a slow variation of the ratio due to aging effects. As a result, the circuit gradually drifts out of specification during usage.
To avoid aging problems, it is known to use a trimming geometry in which resistive links are either totally cut, or left undisturbed. The infinite resistance of a cut link is unaffected by aging. Known techniques which make use of this property include a set of resistive links which are connected in a parallel geometry. However, if N links are used, the resolution of the trim is only 1/N. Trim resolutions can be increased by using binary-weighted links with values in the ratio of 1, 2, 4, etc. A problem with these parallel-connected geometry arrangements is that although the spacings between conductance values is uniform, spacing between resistance values is not. Geometries of this type also require a large amount of area on the integrated circuit because sufficient space, typically on the order of at least twelve microns, must separate each link so that a laser beam can be inserted and withdrawn between individual links.
An integrated circuit resistor which includes a plurality of parallel sections is illustrated in the Brokaw U.S. Pat. No. 4,586,019. The resistor is formed as a plurality of parallel strips to match the sensitivities of different-sized resistors to relative changes in resistance resulting from changes in width.
Clearly, there is a continuing need for improved resistor networks and methods for trimming the resistance value thereof. The resistor network and method will preferably be applicable to both resistor matching applications as well as the selection of absolute resistor values. The resistor network must be compact so as to utilize little space on an integrated circuit, and yet permit high resolution (i.e., small intervals between adjacent trimmed values). A resistor network having these characteristics would be especially desirable if it were not susceptible to the effects of "aging" due to annealing affects. Other desirable characteristics of such a resistor network would be the capability of obtaining a uniform trim sensitivity, and suitability for use with traditional "adaptive" or "continuous" trim algorithms which do not assume prior knowledge or measurability of the desired resistor value.